NewService creates a GitHub-backed notifications.Service using given GitHub clients.
At this time it infers the current user from the client (its authentication info),
and cannot be used to serve multiple users.

Caching can't be used for Activity.ListNotifications until GitHub REST API v3 fixes the
odd behavior of returning 304 even when some notifications get marked as read.
Otherwise read notifications remain forever (until a new notification comes in).

This service uses Cache-Control: no-cache request header to disable caching.